Flexible Optical Metasurfaces Promise "Smart" Contact Lenses

For first time, flexible, mechanically tunable, dielectric resonators are developed for metasurfaces

By Dexter Johnson

Illustration: RMIT/The University of Adelaide

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The name of the game in optical metasurfaces is shortening the wavelengths of light. This yields devices that can manipulate light for information processing and also reduce the bulk of the devices based on traditional optics.

Metasurfaces have been pretty good at offering small, flat features, but the integrated metallic resonators they use to filter light according to specified frequencies have lacked efficiency. It was thought that dielectric resonators were an attractive alternative, but they present another problem: though fairly efficient, their frequency-filtering is hard to fine-tune.

Now researchers at RMIT University and the University of Adelaide have delivered the best of both worlds with a dielectric resonator that can be mechanically tuned. The property that makes these mechanically tunable dielectric resonators especially attractive is that they are embedded in a biocompatible polymer that renders them flexible.

The tuning action is quite simple, explains Philipp Gutruf, a professor at RMIT and who led the research, in an e-mail interview. “The frequency at which they can filter light can be changed by stretching the elastic material that the resonators are embedded in.” Changing the distance between the individual resonators changes their interaction, thereby changing the entire unit’s filtering properties.

According to the researchers, the technology could lead to high-tech contact lenses capable of filtering out harmful optical radiation like glare without interfering with vision. “In a more advanced version,” says an RMIT press release, tunable contacts could “transmit data and gather live vital information or even show information like a head-up display.” (It could also result in even tinier cellphones; tunable lenses would also make flexible ultrathin smartphone cameras possible.)

While Gutruf concedes that dielectric resonators have been demonstrated before this latest research, which is described in the journal ACS Nano, this marks the first time we’ve seen a mechanically tunable version in a biocompatible elastomeric substrate. The key technological achievement made by the Australian researchers was combining high temperature processed titanium dioxide (an important ingredient in sunscreen) with the rubber-like material, and achieving nanoscale features.

Gutruf and his colleagues have developed a functional device on the lab scale that has been characterized and has been shown to work efficiently. The results also match theoretical predictions.

Still, he offers a disclaimer: “We have not demonstrated a contact lens as such.”

Why the cautious optimism? The biocompatible materials the team used are generally used in contact lenses, but there are a few more engineering challenges that must be overcome in order to get from here to the contact lens they envision. Among them is the the development of commercially viable fabrication techniques for creating nanometer-scale resonators in an elastomeric substrate as well as the accompanying stretchable electronics.

The Australian researchers are already working on overcoming those hurdles. The next step in the research, according to Gutruf, is to show a fully functional contact lens and to demonstrate other tunable functionality with this technique.